Mass Effect 2
The M-920 Cain is a portable particle accelerator surrounding an array of dust-form element zero chambers. This weapon prototype subjects its eezo to extreme positive and negative currents to project mass effect fields. By increasing and decreasing mass, the fields shear the target's mass the way disruptor torpedoes do. The shearing fields collide ambient materials at such high speeds, they create mushroom clouds, an effect otherwise impossible on the small scale. The weapon induces neither fission nor fusion in non-nuclear targets, and its own nuclear reactions are shielded by lead alloys. The M-920 Cain uses graphite rods as neutron moderators, which require frequent replacement to sustain power. Fortunately, most heavy weapon ammunition can be re-fabricated via omni-tool into graphite rods.
Mass Effect 3
The M-920 Cain is a portable particle accelerator surrounding an array of dust-form element zero chambers. By subjecting its eezo chambers to extreme positive and negative currents fueled by antimatter reactions, the weapon projects mass effect fields that shear away at the target. The fields warp ambient materials with such explosive force that the impact produces a mushroom cloud. This has led Alliance marines to call the Cain a "nuke gun," though its detonations do not in fact produce fallout.
The M-920 uses graphite rods as neutron moderators, but they require frequent replacement to sustain power. Fortunately, the omni-tool can refabricate most heavy weapon ammunition into graphite rods. The amount of charge-up time is understandable as the weapon is a juggernaut capable of unstoppable destructive power.